<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>新增边框属性</title>
  <style>
    div {
      width: 400px;
      height: 400px;
      border: 2px solid black;
      margin: 0 auto;
      /* 边框圆角 */
      border-radius: 200px;
      /* 边框外轮廓 */
      outline: 2px solid red;
    }
  </style>
</head>
<body>
  <div></div>
</body>
</html>